Vertebrates recognize and discriminate thousands of odorants of diverse molecular structure. How is this process of molecular recognition accomplished? The identification of an odorant's chemical structure is thought to occur through the combinatorial integration from multiple odorant receptors, each tuned to recognize different molecular features. Thus, by elucidating the molecular specificities of the odorant receptors we will gain a better understanding how information is processed in the olfactory system. In addition, knowledge of the molecular principles underlying odorant recognition will illuminate how these receptors are tuned to bind and discriminate odorous ligands. In this application, we focus on the properties of the "C family" G protein-coupled receptors expressed in the vertebrate olfactory system. This receptor family includes the putative pheromone-sensing receptors of the mammalian vomeronasal system and the amino acid sensing receptors of the fish olfactory system. Our studies comprise three major lines of investigation using the fish as a model system: (1) We will apply and refine computational modeling approaches to elucidate the ligand-receptor interactions in two olfactory amino acid receptors expressed in the fish olfactory system. A novel "virtual high-throughput" computational screening protocol will also be implemented to identify high- affinity ligands for these receptors. Such ligands will provide useful tools for probing the molecular architecture of the receptors'ligand binding pockets. These studies will advance our understanding of the molecular determinants underlying ligand recognition in this class of chemosensory receptor. (2) Using recently developed heterologous cell-based and in vivo assay systems, we will characterize the ligand tuning properties of other members of the olfactory C family receptor repertoire. Elucidation of the receptive field properties of individual receptors in turn will allow an understanding of how multiple receptors are used in combination to recognize and discriminate odorants. (3) We will study the role of receptor multimerization in olfactory receptor trafficking and function. We will test the hypothesis that heteromeric receptor interactions are required for the localization of functional receptors to the sensory neuron's apical dendrite - the site of interaction with odorants in the external environment. These studies will establish what role, if any, receptor- receptor interactions play in the formation of functional receptors in the olfactory sensory neuron. Together our studies will help to elucidate the molecular and cellular mechanisms of olfactory coding. PUBLIC HEALTH RELEVANCE The olfactory system receives and interprets chemical cues from the environment that regulate feeding, reproduction, and other social behaviors. These chemical cues are detected by receptors expressed in the primary sensory neurons of the nose. The studies proposed in the present application will investigate the principles underlying chemical recognition by olfactory receptors, and will illuminate how these receptors regulate important physiologic and behavioral processes in humans and non-human animals.